The present invention relates to an intervertebral disk prosthesis which is placed between the upper end plate and the lower end plate of the vertebral bodies of the overlying and underlying vertebrae of a spinal column.
German patent DE 2263842 discloses an intervertebral disk prosthesis comprising an upper plate and a lower plate which are provided respectively on their inner face with a cavity in the shape of a portion of a sphere and with a spherical core placed between the plates and collaborating with the cavities that have the same profile.
The upper plate and the lower plate are joined together and around the spherical core by a peripheral element made of a soft material such as plastic.
French patent FR 76 37174 (2 372 622) discloses an intervertebral disk prosthesis intended for all discal afflictions ensuing from crushing, displacement or deterioration of all kinds of vertebral bodies.
This disk prosthesis consists of a spherical element, forming a horizontal plate, which comprises semispherical sectors protruding from each side of the horizontal surface of the element.
The spherical sectors are placed at the center of the plate at equal distances from the anterior, posterior and lateral edges of the element.
The spherical element is placed between the upper end plate and the lower end plate of the vertebral bodies of the overlying and underlying vertebrae of the spinal column, so that the spherical sectors rest directly against said end plates of the vertebrae.
It is noted that the prosthesis described in patent FR 76 37174 has certain disadvantages regarding the bearing surface via which the spherical element bears against the upper end plate and the lower end plate of the vertebral bodies.
What happens is that the spherical sectors of the element bear directly against the bony end plates of the vertebral bodies, leading to these, on account of the spherical shape, digging into the bony end plates and making it possible for the prosthesis to work.
Patent EP 0 176 728 discloses an intervertebral disk prosthesis comprising an upper plate and a lower plate which are equipped respectively with a middle part with a concave profile intended to accommodate a bi-convex spacing piece with the same radius of curvature.
The spacing piece comprises a flat annular guide edge which is surrounded by an annular bulge which prevents said part from slipping or sliding out of the terminal plates.
It is noted that the intervertebral disk prosthesis described in patent EP 0 176 728 has certain disadvantages as regards operation of the articular connection between the spacer piece and the concave middle parts of each, upper and lower, plate. What happens is that this articular connection is not protected from the fibrous tissues surrounding the prosthesis, which may enter the latter and become fixed to the moving parts, impeding its correct operation.
The main function of the intervertebral disk prosthesis according to the present invention are:
to restore the mobility between the overlying and underlying levels of the vertebrae of a spinal column using a two-ball or bi-concave connection,
to restrict and control movements in bending, inclination and rotation of the upper plate and of the lower plate one with respect to the other and about the spacing piece or core,
to prevent the fibrous tissues or the pseudo-synovial fibrous ganglion from attaching to the moving parts.
The intervertebral disk prosthesis according to the present invention comprises an upper plate and a lower plate which are anchored respectively to the upper end plate and the lower end plate of the vertebral bodies of the overlying and underlying vertebrae of a spinal column and a central core placed between the two plates which has an upper spherical cap and a lower spherical cap collaborating respectively with spherical cavities formed in said upper and lower plates, and an annular element made of a viscoelastic material which is centered around the core and which comes into contact with bosses provided on each inner face of the upper plate and of the lower plate so as to restrict and control the movements in bending, inclination and rotation of the upper plate and of the lower plate one with respect to the other and about the core, as to ensure stability in the positioning of the core between the two plates, and as to avoid fibrous deposits inside the prosthesis.
The intervertebral disk prosthesis according to the present invention comprises an upper plate and a lower plate which has an outer face integral with teeth to allow said plates to become anchored in the respective upper end plate and lower end plate of the vertebral bodies of the overlying and underlying vertebrae.
The intervertebral disk prosthesis according to the present invention has an inner face of the upper plate and of the lower plate which has a spherical cavity delimited by a circular rim which is raised with respect to the horizontal plane of the face.
The intervertebral disk prosthesis according to the present invention has an inner face which has, between the central cavity of spherical profile and the peripheral edge of each plate, bosses.
The intervertebral disk prosthesis according to the present invention has bosses which are arranged in an arc of a circle in such a way as to be centered about the spherical cavity.
The intervertebral disk prosthesis according to the present invention has a central core which has a peripheral ring arranged in a horizontal plane and which collaborates with an internal groove formed in the annular element.
The intervertebral disk prosthesis according to the present invention has an annular element which has a central opening for the passage of the caps of the central core and an internal groove opening into the central opening so as to accommodate the peripheral ring of said core when the disk prosthesis is being assembled.
The intervertebral disk prosthesis according to the present invention has an upper plate and a lower plate which are made of a cobalt/chromium alloy.
The intervertebral disk prosthesis according to the present invention has a central core which is made of a plastics material which has very good slip characteristics, such as polyethylene.
The intervertebral disk prosthesis according to the present invention has an annular element which is made of a viscoelastic material such as biocompatible elastomer for example.